The present invention relates to control apparatus and method for controlling an engaging force of a friction-engagement element such as a lockup clutch adapted to restrict a relative rotation (slip rotational speed) between input and output elements of a torque converter.
A torque converter transmits power between its input and output elements via fluid. Accordingly, the torque converter has a torque-fluctuation absorbing function and a torque increasing function, but has a low transmission efficiency. Therefore, a lockup-type torque converter is nowadays frequently used. In this lockup-type torque converter, the input and output elements of the torque converter is directly connected (the slip rotational speed is made equal to 0) by engaging a lockup clutch under a running condition where the torque-fluctuation absorbing function and torque increasing function are unnecessary. Alternatively, the slip rotation of the torque converter is restricted by a slip control for the lockup clutch under the running condition where the torque-fluctuation absorbing function and torque increasing function are unnecessary.
Particularly, the present invention relates to, for example, a technique for controlling the engaging force of the friction-engagement element such as the above-mentioned lockup clutch so as to cause a differential rotation (slip rotational speed) between input-side and output-side rotational speeds to become a target slip rotational speed, more particularly a technique for controlling the engaging force of the friction-engagement element to cause an actual slip rotational speed of the friction-engagement element to converge to the target slip rotational speed taking a value equal to 0. Japanese Patent Application publication No. 1993(H05)-079557 discloses a previously proposed control apparatus as one example of such an engaging-force control technique for the friction-engagement element.
In order to control the engaging force of friction-engagement element to cause its slip rotational speed to become the target slip rotational speed taking the value equal to 0, it is conceivable that the engaging force of friction-engagement element is brought to its maximum engaging force value within a controllable range of mechanism.
However, the maximum engaging force value of the friction-engagement element is designed at a higher value than its minimum-necessary engaging force value for causing no slip, to have a margin (allowance) against this minimum-necessary engaging force value, in order to prevent the friction-engagement element from slipping. Hence, when the engaging force of the friction-engagement element is reduced from the above-mentioned maximum engaging force value to an engaging force level for the slip control, the slip control of the friction-engagement element cannot be started until the reduction of an amount of engaging force corresponding to the above-mentioned margin has been completed. That is, when the friction-engagement is transferred from in an engaged state where the friction-engagement is being engaged with the maximum engaging force to in a slip control state where the engaging force of the friction-engagement element is being controlled to cause the actual slip rotational speed to become or maintain a given slip rotational-speed value other than 0, the slip control of the friction-engagement element is started after the reduction of the engaging-force amount corresponding to the above-mentioned margin has been completed. Hence, there is a problem that a response delay of the slip control becomes great by that amount.
In order to solve this problem, in the disclosure of above Japanese Patent Application, when the lockup clutch of torque converter is made under the engaged state where its slip rotational speed is equal to 0 during a coasting running of the vehicle (with a release of accelerator pedal), the engaging force of the lockup clutch is reduced if the slip rotation does not occur and is increased if the slip rotation occurs by means of a learning control. That is, an engaging-force control technique of the friction-engagement element is disclosed in which the engaging force is determined at values obtained by the learning control each of which is a borderline value between the occurrence and non occurrence of slip rotation.
According to this engaging-force control technique of the friction-engagement element, the engaging force of the friction-engagement element is set at a borderline value (boundary) obtained through the learning by judging whether or not the slip rotation is caused. Hence, when the friction-engagement element is brought from its engaged state to its slip control state, it is only necessary to reduce the engaging force of the friction-engagement element from the learning value smaller than the above-mentioned maximum engaging-force value to an engaging force level for the slip control. Thus, the above-mentioned problem that the response delay of slip control is great can be solved.